As one of the scan schemes in an X-ray computed tomography apparatus, there is available a scheme of scanning the heart in synchronism with signals from an electrocardiograph (i.e., ECG gated scanning). In ECG gated scanning, to obtain optimal time resolution, scanning is performed for only a period corresponding to a preset beat count. A beat count is decided in accordance with the heart rate (beat count per unit time) or the like of a subject which is measured before a scan. In some case, the heart rate becomes lower than that before a scan, and projection data which can be reconstructed is acquired in one pulsation (beat). However, a scan does not finish unless the scan time elapses. That is, unnecessary X-rays which are not used for reconstruction have been applied. Assume that a short scheduled scan time is set. In this case, when the heart rate becomes higher than that at the time of scanning, the scan finishes before the acquisition of projection data necessary and sufficient for reconstruction. This makes it necessary to set a relatively long scan time.
It is an object to provide an X-ray computed tomography apparatus which can improve the scan efficiency concerning ECG gated scanning.